Purification of tar acids



Oct. 9, 1956 a. w. JONES ET AL PURIFICATION oF TAR ACIDS 2 Sheets-Sheet2 Filed Nov. 27, 1953 wJOZmIa mmrw wJOmmOI I52Min.

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ATTORNEY United States Patent O PURIFICATION F TAR ACIDS ApplicationNovember 27, 1953, 'Serial No. 394,544 3 Claims. (Cl. 26d- 627) Thepresent invention relates to the production of pure tar acids and moreparticularly to an improved process for recovering pure tar acids bydouble solvent extractive methods.

Commercial tar acids include phenols, cresols, xylenols and certainhigher boiling phenols such as ethyl phenols and triniethylphenols. Mostof these tar acids boil in the range of 180 to 240 C. and are marketedas pure compounds or as closely boiling distillate fractions. Tar acidsoccur in coal tars, coal hydrogenation liquid products, shale oils,petroleum oils and the like. They are generally recovered as a tar acidoil by distillation of the source material and appear in mixture withneutral oils, tar bases and other contaminants, such as sulfur compoundsand certain carboxylic acids. Tars produced by the low temperaturecarbonization of Pittsburgh seam coal contain in their tar acidsdistillate fractions about 40 to 60 percent tar acids, about 40 to 60percent neutral oils and less than about 5 percent tar bases. The exactcomposition depends upon the particular coal as well as upon the methodof carbonization. However it is apparent that the principal separationproblem in connection with tar acids is the removal of the neutral oils.Commercial specifications for tar acids frequently prescribe the neutraloil content to be less than 0.1 weight percent. p

The term tar acid oil denes any distillate fraction containing tar acidsin mixture with neutral oils. The lowest boiling tar acid, phenol, boilsat 180 C., although azeotropes of phenol boil as low as 160 C. rhexylenols generally boil below 240 C. Other tar acids, however, haveboiling ranges as high as about 300 C. Thus tar acid oil is a distillatefraction boiling anywhere in the range of about 160 C. through about 300C. To recover the commercially valuable phenol, cresols and xylenols,however, only the fractions boiling from about 160 C. to about 240 C.are required. The present process is applicable to any tar acid oilboiling in the range of about 160 to 300 C., provided the lower boilingtar acids are present.

A solvent extraction process for separating tar acids from neutral oilshas been developed and is described in U. S. patent application S. N.184,474, entitled Refining of Tar Acid Oil by Martin B. Neuworth andEverett Corin, filed September 12, 1950, now U. S. Patent 2,666,- 796issued January 19, i954 as a continuation-in-part of U. S. patentapplication S. N. 110,932, filed August 18, 1949, new abandoned. Toseparate tar acids from tar acid oil according to the process ofapplication S. N. 184,474 supra, the tar acid oil is contacted in acenterfeed extraction column with two solvents: (l) a parafiinichydrocarbon solvent (naphtha) having a boiling range of 60 to 130 C. anda density of less than 0.8; and (2) aqueous methanol containing 55 to 75weight percent methanol. The tar acids are selectively recovered in theaqueous methanol whereas the neutral oils are selectively recovered inthe paraliinic solvent. The total tar acids recovered from this processsatisfy the specifications of commerce, i. e. the neutral oilcontamination of the product tar acids is less than 0.1 weight percent.

However, upon precise fractionation of the tar acids recovered in theprocess of U. S. patent application S. N. 184,474, it has been foundthat the residual neutral oil contaminant is not uniformly distributedthroughout the distillate fractions. Instead, the neutral oils arepreferentially concentrated in the lowest boiling distillate fractionscontaining the phenol (boiling point 181 C.) and ortho-cresol (boilingpoint 191 C.). In fact the neutral oil contamination of these lowestboiling fractions is as high as 0.2 weight percent and higher. Todispose of these lower boiling tar acids as a premium product, it isdesirable that the neutral oil contamination be reduced.

We have found that these lowest boiling tar acids can be separated fromthe neutral oils which accompany them in distillate fractions bytreating the fractions in a twosolvent extraction process similar tothat shown and described in U. S. patent application S. N. 184,474supra. However the aqueous methanol solvent for these lower boiling taracids should contain from 40 to 55 weight percent methanol where it isdesired to separate these low boiling tar acid oils into high purity taracids, i. e. having less than 0.1 weight percent neutral oilcontamination, for example.

Accordingly, to recover high purity tar acids from the total tar acidoil, the feed material is subjected to a first fractionation to separateit into 1) a low boiling distillate fraction having an end boiling pointof about to 210 C., containing all the phenol and some of the cresolsfrom the tar acid oil feedstock and (2) a higher boiling distilatefraction having an initial boiling point of about 180 to 210 C., andcontaining some orthocresol but no phenoi from the tar acid oilfeedstock. The higher boiling fraction thereupon is treated inaccordance with the solvent extraction process of U. S. patentapplication S. N. 184,474 supra. The lower boiling fraction however istreated in a solvent extraction process similar to that set forth in U.S. patent application S. N. 184,474 supra, but differing in that thepolar solvent is a 40 to 55 weight percent solution of aqueous methanol.Pure tar acids having low residual neutral oil contamination arerecovered from each solvent extractive process.

For a full explanation of the present invention, its objects andadvantages, reference should be had to the following description and tothe accompanying drawings in which:

Figure 1 is a schematic ow sheet showing the preferred embodiment of thepresent invention;

Figure 2 is a graphical illustration of the purity of tar acids producedwithout pre-fractionation;

Figure 3 is a graphical illustration of the purity of tar acids producedaccording to the present invention; and

Figure 4 is a schematic flow sheet showing a modification of the processof our invention for recovering waterwhite tar acids, free of neutraloils, free of tar bases and free of foulcmelling carboxylic acids.

Referring to Figure 1, the feed material is tar acid oil having aboiling range of about 160 to about 240 C. for this example. The taracid oil is introduced into a primary fractionation zone 10 where it isseparated into two distillate fractions. The cut point between the twofractions in general ranges from about 180 to about 200 C. and isdetermined by the composition of the tarV acid oil feedstock and thedesired products. The lower boiling distilate from the fractionationzone 10 should contain all the phenol and some of the cresols in the taracid oil feedstock. The light fraction containing the phenols comprisesless than 20 percent of the 160 to 240 C. tar acid oil from tar producedby low temperature carbonization of bituminous coal. The bottoms fromthe fractionation zone 10 should contain some cresols but no phenol. Forthe purposes of illustration, the fractionation zone of Figure 1 isshown as operating at a cut point of 190 C.

The lower boiling distillate fraction from the fractionation zone passesthrough a conduit 11 to a solvent extraction Zone 12. The extractionprocess carried out in the zone 12 is similar to that described in U. S.patent application S. N. 184,474 supra, but differs in that the methanolconcentration of the aqueous polar solvent is about 40 to 55 weightpercent. A paraiiinic naphtha fraction boiling in the range of 60 to 130C. and having a density of less than 0.8 is introduced into the bottomof the extraction zone 12. Aqueous methanol having a methanolconcentration of about 40 to 55 weight percent is introduced into thetop of the extraction zone 12. For every volume of tar acid oil fedthrough the conduit l11, from 0.5 to 5.0 volumes of aqueous methanol andfrom 0.5 to 5.0 volumes of paraiiinic naphtha should be used. The ratioof naphtha to aqueous methanol, moreover, should be from about 0.25 toabout 4.0.

The parainic naphtha passes upwardly through the ex traction zone 12 andselectively absorbs the neutral oil constituents yof the tar acid oil.The aqueous methanol passes downwardly through the extraction zone 12and selectively absorbs the tar acid constituents of the tar acid oil.Neutral oils in naphtha solvent are recovered through a conduit 17;aqueous methanol and tar acids are recovered through a conduit 18.Effluent tar acids from the extraction Zone 12 possess low residualneutral oil contamination. In general these tar acids comprise all thephenol in the tar acid oil feedstock together with some of the cresols.If desired, the aqueous methanol solution of pure tar acids in conduit18 can be stripped of methanol and the remaining tar acids may beseparately recovered. In Figure 1, however, for the purposes ofillustration, the purified tar acids in aqeous methanol solution, arerecombined with higher boiling tar acids for further refining.

The higher boiling tar acid oil fraction from the fractionation Zone 10passes through the conduit 13 to a solvent extraction zone 14 where itis treated in accordance with the process set forth in U. S. patentapplica tion S. N. 184,474 supra. Aqueous methanol having aconcentration of 55 to 75 weight percent and a paraiiinic naphtha areemployed as solvents. The naphtha passes upwardly through the extractionZone and selectively absorbs the neutral oil constituents of thefeedstock. The aqueous methanol passes downwardly through the extractionzone and selectively absorbs the tar acid constituents. Neutral oils innaphtha solvent are recovered through the conduit 15. Effluent tar acidsfrom the extraction zone 14 possess a low residual neutral oilcontamination, and comprise generally all of the tar acids from thefeedstock except for the phenol and a portion of the cresols. These taracids may be separately recovered from their aqueous methanol solvent ormay be recombined as shown in Figure l and fractionated as a blend ofall tar acids.

The two streams 16 and 18 contain substantially all the tar acids fromthe tar acid oil feedstock in aqueous methanol solution. Aqueousmethanol is stripped from the tar acids in a stripping zone 20 andrecovered for reuse as solvent through the conduit 21. Tar acids, freedof solvent, are recovered from the stripper 20 through a conduit 22 andsent to a precise fractionation zone 23 where closely boiling distillatefractions of the tar acids are recovered as final products.

To show the improvement resulting from the present invention, theneutral oil contamination of the distillate fractions of purified taracids is plotted in Figures 2 and 3 against the percentage of the taracids distilled. For

IFigure 2, raw tar acid oil was treated in accordance with U. S. patentapplication S. N. 184,474 and the purified tar acids contained aboutlive percent water but had less than 0.1 weight percent neutral oilcontamination. As indicated in Figure 2, the neutral oil contaminantsappear to be concentrated in the lowest boiling tar acids, i. e. in thelowest boiling ten percent of the purified tar acids. Throughout thephenol and ortho-cresol boiling range the neutral oil contamination wasof the order of 0.2 weight percent; the initial fraction of the taracids had as much as 2.0 weight percent contamination.

When the same raw tar acid oil was treated according to the presentinvention, a lower neutral oil contamination of the lower boiling taracids resulted as indicated in Figure 3. Even the lowest boiling taracids had a residual neutral oil contamination of less than 0.1 weightpercent. Thus, by employing the present invention it is possible toproduce pure tar acids which satisfy many commercial specifications overthe entire boiling range, i. e., the neutral oil contamination is lessthan 0.1 weight percent.

Separation of the lowest boiling tar acids from tar acid oil by thedouble solvent extraction process results in high yields of tar acidsthroughout the range of aqueous methanol concentrations from 40 to 75weight percent. However only in the range of 40 to 55 percent aqueousmethanol can high purity tar acids be recovered from the lower boilingtar acid oil. Similarly, the higher boiling tar acids can be recoveredin high yield and high purity only with aqueous methanol having aconcentration of 55 to 75 percent. Treatment of the higher boiling taracid oils with an aqueous methanol solvent having a concentration of 40to 55 percent would produce pure tar acids in low yields, since many taracids would appear in the naphtha phase.

Another advantage resulting from pre-fractionation of the tar acid oilis a saving in intermediate refining processes for the tar acids. Tounderstand this intermediate saving, reference should be had to Figure 4which is a schematic flow sheet of an overall tar acids refining systemaccording to the present invention.

As shown in Figure 4, tar acid oil having a boiling range of 160 to 240C. from a source 30 is pre-fractionated in a distillation zone 31 intotwo distillates, one boiling from 160 to about 190 C. and the otherboiling from about 190 to 240 C. The exact temperature of thefractionation is determined by the composition of the tar acid oil. Theoverhead fraction should contain substantially all the phenol Whereasthe bottoms fraction should contain substantially no phenol. The heavierfraction (l-240 C.) is treated in a solvent extraction Zone 32 inaccordance with the process of S. N. 184,474 as previously described.

The to 190 C. overhead fraction is treated in a solvent extraction zone33 with parainic naphtha and 40 to 55 weight percent aqueous methanol.The aqueous methanol extract, containing the 160 to 190 C. tar acids, issubstantially free of neutral oils, but does contain some carboxylicacids which create foul odors and also some tar bases which must beremoved. An anion exchange process for removing the foul smellingcarboxylic acids has been described in copending U. S. patentapplication S. N. 276,991 by Martin B. Neuworth, now U. S. Patent2,734,925 issued Feb. 14, 1956, entitled Puriication of Low Boiling TarAcids and filed on March 17, 1952 as a continuation-impart of U. S.patent application S. N. 215,214, filed March 13, 1951, now abandoned.According to the process of S. N. 276,991, foul smelling carboxyliccontaminants may be removed from tar acids by passing an aqueousmethanolic solution of tar acids through a vessel containing certainanion exchange materials. A vessel 34 containing anion exchange materialis provided for treating the light fraction of tar acids in aqueousmethanol solvent. We have found that the foul-odor-producing materialsalso are concentrated in the light fraction of the tar acids. Hence itis not required that the heavy fraction (-240 C.) be treated in theanion exchange Vessel corresponding to vessel 34.

The effectiveness Vof the anion exchange treatment for the lower boilingtar acids furthermore is improved by the fact that the higher boilingtar acids do not contact the anion exchange materials. The high boilingtar acids and particularly those boiling above 240 C., have a tendencyto clog the pores of the anion` exchange material, diminishing itsactivity and reducing its effective life. Without the pre-fractionationafforded by the present invention, it would be required to treat theentire 160-240" C. fraction of tar acids in an anion exchange zone.According to the present invention, only the lighter fraction, whichcomprises perhaps ten percent of the weight of the tar acid oil, must betreated with the anion exchange process. Accordingly only sutiicientanion exchange capacity for about ten weight percent of the total taracid oil is required. The capital investment in such a tar acid oilrelinery is reduced, Moreover the relatively expensive anion exchangematerial is not subjected to contact with the higher boiling tar acidswhich have a tendency to sludge the anion exchange material.

Tar bases can be removed from the tar acids by a cation exchange processdescribed in U. S. patent application S. N. 382,866 by Everett Gorin,Martin B. Neuworth and Benjamin W. Jones, entitled Separation of TarAcids and Tar Bases, filed on September 28, 1953 as acontinuation-in-part of U. S. patent application S. N. 215,376 by thesame inventors and having the same title, tiled on March 13, 1951, andnow abandoned.

Since the tar base contaminants are distributed throughout the entireboiling range of the tar acid oil, it is necessary that both fractionsbe treated by a cation exchange process in order to remove the tar basecontamination. The light fraction is combined with the heavy fractionand the mixture is treated in a cation exchange vessel 35. Eiiuent fromthe vessel 35 contains substantially no tar bases, substantially noodor-producing carboxylic acids, and essentially no neutral oilcontaminants. Aqueous methanol solvent is removed from the puriiied taracids in a stripper 36. The resulting tar acids are separated intoclosely boiling marketable fractions in a precise fractionaldistillation zone 37.

By pre-fractionating the tar acid oil and separately treating the twofractions by solvent extraction, it is possible to produce tar acidswhich satisfy the commerciel specifications throughout the entiredistillate range. Moreover it is possible to eliminate the foul-smellingcarboxylic contaminants from the tar acids products by treating only thelighter fraction with anion exchange materials; increased life of theexchange material results since contact between the heavier tar acidsand the anion exchange materials is avoided.

Now, according to the provisions ofthe patent statutes, we haveexplained the principle, preferred construction, and mode of operationof our invention and have illustrated and described what we now considerto represent its best embodiment. However, we desire to have itunderstand that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically illustrated anddescribed.

We claim:

1. A method for refining low boiling; tar acid oil to recover pure taracids which comprises separating the low boiling tar acid oil into twodistillate fractions, a iirst containing substantially all the phenoland some cresol and a second containing some cresol but substantially nophenol, treating said first fraction in a first countercurrent,center-feed, double solvent extraction system using paraftinic naphthahaving a density less than 0.8 and 40 to 55 percent aqueous methanol assolvents, treating said second fraction in a second countercurrent,center-feed, double solvent extraction system using parafiinic naphthahaving a density less than 0.8 and to 75 percent aqueous methanol assolvents, recovering the aqueous methanol extracts from both solventextraction systems and recovering from said aqueous methanol extractstar acids containing substantially no neutral oils.

2. The method of claim 1 in which the tar acid oil is derived from tarproduced by the low temperature carbonization of bituminous coal.

3. A method for recovering tar acids from tar acid oil boiling in therange to 240 C. which comprises separating the tar acid oil into twodistillate fractions, the first having an end boiling point in the rangeto 200 C. and the second distillate fraction having an initial boilingpoint in the range 180 to 200 C., treatling the low boiling fraction ina first countercurrent,

center-feed, double solvent extraction system using 40 to 55 percentaqueous methanol and parainic naphtha having a density less than 0.8 assolvents, treating the higher boiling fraction in a secondcountercuirrent, centerfeed, double solvent extraction system using 55to 75 percent aqueous methanol and parainc naphtha having a density lessthan 0.8 as solvents, recovering and combining aqueous methanol extractsfrom each solvent extraction system, and recovering from the combinedextracts tar acids substantially free of neutral oils.

References Cited in the tile of this patent UNITED STATES PATENTS1,077,287 McDougall et al. Nov. 4, 1913 1,528,313 Weindel Mar. 3, 19251,582,512 Crawford Apr. 27, 1926 1,934,861 Karpati et al Nov. 14, 19332,206,198 Molinari July 2, 1940 2,334,691 Andersen Nov. 23, 19432,666,796 Gorin et al. Ian. 19, 1954

3. A METHOD FOR RECOVERING TAR ACIDS FROM TAR ACID OIL BOILING IN THE RANGE 160 TO 240* C. WHICH COMPRISES SEPARATING THE TAR ACID OIL INTO TWO DISTILLATE FRACTIONS, THE FIRST HAVING AN END BOILING POINT IN THE RANGE 180 TO 200* C. AND THE SECOND DISTILLATE FRACTION HAVING AN INITIAL BOILING POINT IN THE RANGE 180 TO 200* C., TREATING THE LOW BOILING FRACTION IN A FIRST COUNTERCURRENT, CENTER-FEED, DOUBLE SOLVENT EXTRACTION SYSTEM USING 40 TO 55 PERCENT AQUEOSU METHANOL AND PARAFFINIC NAPHTHA HAVING A DENSITY LESS THAN 0.8 AS SOLVENTS, TREATING THE HIGHER BOILING FRACTION IN A SECOND COUNTERCURRENT, CENTER- 